From the Oxford English Dictionary, XI.1099:

†’ mortalist. Obs. [f. MORTAL a. +IST.] One who believes the soul to be mortal [see quot.]
1636 R. BAILLIE Anabaptism (1647) The Familistick Anabaptists…are not only for the sleeping of the soul at death, but its annihilation…At first these mortalists did grant a resurrection of the body…but now..they deny any resurrection after death, all life either of body or soul.
1757 MRS. GRIFFITH Lett. Henry & Frances (1767) II. 26 A mortalist Deist sinks into an Atheist.

Curious that so useful a concept should be designated by so obscure a word. Hat tip to Peter Heinegg, Better than Both: The Case for Pessimism for drawing my attention to it.